The web source section's visibility was bound to SelectedElement.Type
via a converter, but WebcamSceneConfig has no Type property — the WPF
binding path didn't resolve correctly and the section leaked through.
Added IsWebSource virtual property to SceneElement (mirrors existing
IsWebcam pattern), overridden in Source to return
Type == SourceType.WebSource. Section visibility now binds directly to
SelectedElement.IsWebSource via BoolToVis.
1c of 7-issue polish batch. 207 tests, 0 warnings.